Israeli Prime Minister Benjamin Netanyahu said Tuesday that German-supplied submarines were "very important" for his country's defence, after a media report that Israel was fitting the vessels with nuclear warheads.

"Germany has underlined its commitment to Israel's security in particular with the sale of another submarine," Netanyahu told the daily Bild. "It is a very important addition to our national security."

Der Spiegel reported in its Monday issue that Israel was arming submarines provided and largely financed by Germany with nuclear-tipped cruise missiles with the knowledge of the German government.

Germany has already supplied Israel with the three Dolphin-class submarines in question and another three are to be delivered by 2017.

Israel is the Middle East's sole nuclear-armed power, though it has never declared its nuclear weapons.

The report said Germany hoped to see Israeli concessions on settlements on Palestinian land and approval for the completion of a sewage treatment plant in the Gaza Strip in exchange for the assistance.

It prompted calls by the German opposition for an explanation from Chancellor Angela Merkel's government and accusations that it had little to show for its help to Israel.

Government spokesman Steffen Seibert said Monday that he would not "speculate" about Israel's nuclear arsenal but indicated that Germany had not stipulated what Israel could do with its submarines once it took delivery.

"I cannot confirm there are any such clauses," he said.

Tensions between Germany and Israel flared in April when Nobel prize-winning German author Gunter Grass published an inflammatory poem warning that a nuclear-armed Israel "could wipe out the Iranian people (with a) first strike."

Merkel has also sharply criticised Netanyahu's pro-settlements policy.

Asked about a poll showing that 70 percent of Germans believed Israel pursued its own interests without consideration for other peoples, Netanyahu said he saw it as an effect of Israel "being slandered day-in and day-out".

"In Germany, with its special history and special ties to Israel, this is particularly unfortunate," he said.